JICA Study Tour - Day 2

Takiya River discharge measurement (H = 0.65m, Q = 3.45 m3/s)

JICA Study Tour - Day 1

Tainai River irrigation headworks

Arakawa River from Maruyama Bridge, Sekikawa Village

Biotope pond for dragonflies alongside Arakawa River

Arakawa River irrigation headworks

Upstream view from Arakawa irrigation headworks

Spring rains

Today over 70 mm of rainfall occurred with a maximum intensity of nearly 20 mm/h (Miomote Amedas Station). This is relatively heavy rainfall for May, which on average receives 150 mm of precipitation for the whole month (average for 1981-2010). This month has received 162 mm to date. When we arrived at the gauging station the river stage was steady at around 1 m, which is too high to take a discharge measurement by the wading method.

A visit to Miomote Dam showed that there was a large spillway discharge. The photo below also shows the hydro-electric generating plant on the left.

Snowmelt season runoff

Stage = 57.25 cm, Q = 3.24 m3/s, Vmax = 0.94 m/s

Snowmelt season in full swing today with fine weather melt conditions. Stage starts to rise about 11am, and peaks about 6pm. Maximum velocities in the centre of the channel reaching nearly 1 m/s making it hard to hold the current meter in position. Unsteady flow and standing waves make it difficult to estimate the flow depth as it constantly fluctuates. However, despite the moderately high discharge level, the water remains extremely clear with no visible sediment load.

The hydrograph above shows a period of fine weather melt during late March where we can see the diurnal melt pattern rising day by day. In contrast, the larger and more abrupt peak flow on April 15 is a rain-on-snow event due to 54 mm of precipitation with a maximum intensity of 12 mm/h.

Final snow survey

Final snow survey of the season today shows continuing melt through March. The snow melts twice as fast in the larch stand compared to the mature cedar stand due to the lack of shading and higher net radiation under the deciduous canopy without leaves.

Snow survey summary, snow water equivalent
Larch: 35 cm (down from 51 cm to 35 cm over 16 days = 1.0 cm/day)
Mature cedar: 28 cm (down from 36 cm to 28 cm over 16 days = 0.5 cm/day)

A discharge measurement was also taken under rainy and very cold conditions to confirm the stage-rating curve after the winter and before the main snowmelt season.

Stage = 57.25 cm, Q = 3.14 m3/s

Log jam removal

Today's main task was to remove the log jam below the gauging station, in addition to downloading the Hobo water level recorder and resetting it under the water by the staff gauge. This was cold work in a stream swollen with snowmelt water. After removing the log jam, the water level immediately dropped by about 55 mm at the gauging station, confirming a backwater effect which had been influencing stage measurement.

March snow survey

Snow survey results showed that the snow water equivalent (SWE) was less than in February, with about 51 cm SWE in the larch stand and about 35 cm SWE in the cedar stand (see photo above).

The snowmelt season has already begun with two runoff events early March, and the winter season also shows several significant runoff events during January and February (see hydrograph below).

The photo above shows a small log jam below the gauging station that was likely having an impact on the gauged water level and the stage-rating curve. The log jam was removed two days later on March 8th, when water levels immediately dropped by about 55 mm at the gauging station.